1. Field of the Invention
The present invention relates, in general, to the removal of moisture from within an electronics enclosure, and in particular to a method and apparatus employing a single heat pump to dehumidify and thermally condition the air within an electronics enclosure.
2. Description of Related Art
Electronic devices, which are cooled to below ambient temperatures and which operate in an environment in which water vapor is present, are subject to condensation problems which may cause corrosion and short circuits. It is therefore desirable to remove water vapor from the environment in which electronic devices operate. While corrosion and short circuiting may be caused by water vapor within the environment surrounding the electronic devices regardless of the device or environment temperatures, the problems intensify for devices operating at lower temperatures, as water vapor may begin to condense on the cooled surfaces of the electronic devices.
As is known, CMOS circuit performance may be improved by reducing the temperatures at which the circuits operate, i.e. the chip junction temperature. As the chip junction temperature decreases, however, the outer surface temperature of the chip package also decreases. The package outer surface includes the electrical interconnections between the package and the next level of assembly, such as a board: the temperature of these interconnections also decreases as chip junction temperature decreases. Heat transfer through these electrical interconnections, therefore, decreases board temperatures. As chip junction temperatures decrease further, eventually the temperature of one or more portions of these exposed surfaces (package outer surface, interconnections, and board) falls below the dew point temperature of the ambient atmosphere surrounding the components. At this point, condensation forms on the module and board surfaces, including electrical interconnections, unless preventive actions are taken. Therefore, dehumidification is highly desirable for environments containing electronic devices operating at temperatures below ambient dew point.
Two basic approaches have been taken to eliminate condensation on cold module surfaces: maintain the external module surfaces at temperatures above room ambient dew point, or condition the atmosphere within the enclosure such that its dew point is lower than the electronic components"" external surface temperatures.
The first method, maintaining module surface temperatures above dew point, may be accomplished by providing sufficient insulation around each module, such that the external surface temperature of the insulation remains above room ambient dew point. This approach is discussed in a United States patent application entitled xe2x80x9cInflatable Sealing System for Low Temperature Electronic Module,xe2x80x9d Ellsworth et al., Ser. No. 09/360,727, having a filing date of Jul. 27, 1999, assigned to the same assignee as the present application and hereby incorporated herein by reference in its entirety, and which is not admitted to be prior art with respect to the present invention by its mention in this Background Section. As the chip temperature decreases, however, more insulation is required around each module to maintain external surface temperatures above the dew point. At some point, as chip temperatures are decreased far below dew point, it may also be necessary to provide auxiliary heaters at the external surfaces of the insulation in order to maintain the module insulation surface temperature above the dew point.
The second method, lowering the dew point of air within the enclosure below the external surface temperature of the cooled modules, prevents condensation without requiring insulation around the modules. This approach is discussed in a United States patent application entitled xe2x80x9cSub-Dew Point Cooling of Electronic Systems,xe2x80x9d Chu et al., Ser. No. 09/281,135, having a filing date of Mar. 29, 1999, assigned to the same assignee as the present application and hereby incorporated herein by reference in its entirety, and which is not admitted to be prior art with respect to the present invention by its mention in this Background Section. In order to lower the dew point temperature of the ambient atmosphere within the enclosure, some method should be employed to remove moisture from the atmosphere within the enclosure, preferably providing the ability to further remove the moisture from within the enclosure itself. Further, in order to decrease the burden on the moisture removal device, it may be desirable in some applications to provide an enclosure that is at least somewhat sealed against entry of ambient air. While sealing the enclosure may not be required in all applications, a well sealed enclosure requires less frequent (i.e. lower duty cycle) operation of the moisture removal device than would be required in a poorly sealed enclosure.
Depending upon the specific application and the moisture removal method used, the enclosure air temperature may be lowered as a byproduct of the moisture removal process. The temperature reduction may be caused by reduced heat transfer between enclosure air and ambient air, as a result of partially sealing the enclosure against ingress of ambient air. Heat transfer from ambient air to enclosure air tends to mitigate the effect of heat transfer from the enclosure air to the cooled surfaces within the enclosure. Alternatively, the temperature reduction may be caused by the use of a cold heat exchanger to dehumidify the enclosure air. For some applications, the presence of other mitigating factors may maintain the enclosure air temperature within acceptable limits. For other applications, however, continued system operation may result in enclosure air temperatures below room ambient dew point, eventually causing the temperature of the enclosure outer surface to drop below room ambient dew point, resulting in the formation of condensation on the enclosure outer surface.
In such applications, two methods have been employed in the art to prevent condensation from forming on the external surfaces of the enclosure. One method involves insulating the enclosure, such that the external surface remains above ambient dew point. A second method involves the use of an auxiliary heater to heat the enclosure air.
For the foregoing reasons, there is a need for methods and devices capable of preventing the formation of condensation on the cooled surfaces of electronic components, without insulating the electronic components or the enclosure. There is, therefore, a need for methods and devices capable of removing water vapor from the atmosphere within an electronics enclosure and further from the enclosure itself, without lowering the temperature of the atmosphere within the enclosure.
The present invention is directed to a method and apparatus for conditioning the air within an electronics enclosure, without external condensation, and without the need to insulate the enclosure or to provide an auxiliary heat source. Toward this end, a recuperative environmental conditioning unit is proposed which dehumidifies the air within the enclosure by causing the air to pass over a heat exchanger in thermal contact with the cold element of a heat pump, and reheats the air prior to returning it to the enclosure by causing the air to pass over a heat exchanger in thermal contact with the hot element of the same heat pump. In this way, moisture is removed from the air within the enclosure, eliminating the need for insulation around the electronic components. Since the dehumidified air is warmed prior to returning to the enclosure, the enclosure temperature remains above the room ambient dew point temperature, eliminating the need to insulate the enclosure. Further, since the air is heated by the hot element of the heat pump, no auxiliary heat sources are required. By using the same heat pump to cool and heat the enclosure air, the heat extracted during cooling is recouped. Finally, the conduit forms a sump or collection area, from which the condensate is purgeable to the external environment.
In one embodiment of the present invention, a closed loop air conduit is formed beneath the electronics enclosure. A heat pump is situated beneath the enclosure, each element of the heat pump having a high thermal conductivity path to the air contained within a different portion of the conduit. An air moving device causes air to circulate from the enclosure into an inlet port of the conduit, through the inlet side of the conduit, through a sump or condensate collection section of the conduit, then through the outlet side and outlet port of the conduit, and finally from the outlet port back to the enclosure. Within the inlet side, the air flows through a heat exchanger in thermal contact with the normally cold element of the heat pump, cooling the air and causing moisture to condense on the normally cold heat exchanger. Within the outlet side, the air flows through a heat exchanger in thermal contact with the normally hot element of the heat pump, heating the air prior to its return to the enclosure. As condensate collects on the normally cold heat exchanger, it falls from the heat exchanger and is collected in the sump portion of the conduit. A mechanism, such as a valve or a wick, which allows periodic removal of the condensate is disposed near the bottom of the sump. A mechanism such as a baffle is provided within the enclosure to cause air entering the enclosure from the conduit outlet port to circulate around the enclosure before reentering the conduit inlet port.
In another embodiment of the present invention, the recuperative environmental conditioning unit is disposed entirely within the enclosure: only the mechanism which removes condensate from the sump area, or some portion of this mechanism, extends outside of the enclosure.
In another embodiment of the present invention, the heat pump device is a vapor compression cycle heat pump. In a preferred embodiment, the heat pump is a thermoelectric device.
In other embodiments of the present invention, air circulation within the enclosure is accomplished by extending the conduit inlet port or the conduit outlet port, or both, so as to cause air to return to the enclosure some distance away from the location where the conduit removes air from the enclosure. In this way, air entering the enclosure from the extended outlet port circulates through the enclosure prior to entering the conduit inlet port, thus eliminating the need for an airflow baffle.
In preferred embodiments of the present invention, control mechanisms are provided to handle certain functions. In one aspect, a humidity or dew point sensor measures the humidity within the enclosure, and a controller monitoring the humidity sensor activates the heat pump and air moving device when humidity within the enclosure exceeds a setpoint. The controller continues to monitor the sensor, deactivating the heat pump and air moving device when the dew point is at or below a setpoint. In another aspect, a control mechanism operates the condensate removal valve when the condensate level reaches an upper threshold.
In a preferred embodiment of the present invention, a defrost mode is provided to remove frost from the normally cold heat exchanger. The defrost mode may be initiated and terminated manually; however, in preferred embodiments a controller monitors a sensor to identify a restricted airflow condition at the normally cold heat exchanger, restricted airflow being indicative of frost accumulation on the heat exchanger. Upon detection of such a condition, the controller initiates the defrost mode. During defrost, the controller reverses the direction of heat flow through the heat pump, causing heat to flow from the normally hot element to the normally cold element. At the same time, the controller initiates a change in airflow at the inlet and outlet sides of the conduit. The conduit inlet side, normally in airflow communication with the conduit inlet port, is now sealed off from the inlet port (and therefore from the enclosure) and put in airflow communication with an ambient inlet vent, the ambient inlet vent being in airflow communication with the ambient air surrounding the enclosure. In analogous fashion, the conduit outlet side, normally in airflow communication with the conduit outlet port, is sealed off from the outlet port (and therefore from the enclosure) and is placed in airflow communication with an ambient outlet vent, the ambient outlet vent being in airflow communication with the ambient air surrounding the enclosure. As a result of these changes, air no longer flows to and from the enclosure, the enclosure being sealed off from the inlet and outlet sides of the conduit. During defrost, room ambient air enters through the ambient inlet vent, into the conduit inlet side, where it flows over the normally cold (now hot) heat exchanger, through the sump, over the normally hot (now cold) heat exchanger, and finally through the outlet side and ambient outlet vent, into the ambient environment. The controller terminates the defrost mode upon determining that the restricted airflow condition has been eliminated. The controller then returns the system to its normal operating state: the conduit is sealed off from the ambient, the conduit inlet side is placed in airflow communication with the inlet port, the conduit outlet side is placed in airflow communication with the outlet port, thereby restoring airflow between the enclosure and the conduit, and the heat pump is again reversed such that the heat pump causes heat to flow from the normally cold element to the normally hot element.
In preferred embodiments employing control mechanisms to initiate dehumidification and defrost, contention is managed by prioritizing the defrost process.
It is therefore an object of the present invention to provide a method and apparatus to remove moisture from within an electronics enclosure, without lowering the temperature of the air within the enclosure.
It is a further object of the present invention to accomplish the moisture removal with a single heat pump, heating the dehumidified air prior to returning the air to the enclosure, without the use of an auxiliary heat source.
It is a further object of the present invention to automate the moisture removal process, by monitoring the moisture levels within the enclosure and activating the conditioning unit when the dew point exceeds a threshold.
It is a further object of the present invention to provide an automatic mechanism for removing condensate from the enclosure.
It is a still further object of the present invention to provide an automated defrost cycle, to remove frost from the normally cold heat pump element, venting the resulting vapor to the external environment.
The recitation herein of a list of desirable objects which are met by various embodiments of the present invention is not meant to imply or suggest that any or all of these objects are present as essential features, either individually or collectively, in the most general embodiment of the present invention or in any of its more specific embodiments.
Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein, and are considered part of the claimed invention.